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            <h1 style="display: none">装饰器模式</h1>
            
            <div class="markdown-body">
              <h2 id="装饰器模式"><a href="#装饰器模式" class="headerlink" title="装饰器模式"></a>装饰器模式</h2><h3 id="使用场景"><a href="#使用场景" class="headerlink" title="使用场景"></a>使用场景</h3><p>当对象不能满足现有的功能时，和类一样遵守“开放封闭”原则。只能扩展，不能修改。</p>
<p>此时可以用装饰器模式，能够给对象添加额外的功能。对象加上它，就像开了挂！</p>
<p>在使用react+dva时，经常会这么写</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-keyword">import</span> React <span class="hljs-keyword">from</span> <span class="hljs-string">&#x27;react&#x27;</span>;
<span class="hljs-keyword">import</span> &#123; connect &#125; <span class="hljs-keyword">from</span> <span class="hljs-string">&quot;react-redux&quot;</span>;
@connect(<span class="hljs-function"><span class="hljs-params">state</span> =&gt;</span> (&#123;
		<span class="hljs-attr">data</span>: state[namespace].data
&#125;))
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Demo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">React</span>.<span class="hljs-title">component</span> </span>&#123;...&#125;</code></pre></div>

<p>第一次见到这种写法时，内心直呼：这是什么牛马！静下心来仔细一想，嗨！这不就是装饰器嘛。</p>
<h3 id="装饰类"><a href="#装饰类" class="headerlink" title="装饰类"></a>装饰类</h3><div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">addStatic</span>(<span class="hljs-params">key, value</span>) </span>&#123;
    <span class="hljs-keyword">return</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">target</span>) </span>&#123;
        target[key] = value;
    &#125;    
&#125;
@addStatic(<span class="hljs-string">&#x27;chip&#x27;</span>, <span class="hljs-string">&#x27;m1&#x27;</span>)
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Mac</span> </span>&#123;&#125;
<span class="hljs-built_in">console</span>.log(Mac.chip); <span class="hljs-comment">// m1</span></code></pre></div>

<p>可以看到addStatic方法有两个参数，目的是给类设置一个静态属性。Mac类本身可没有这个功能喔，完全是装饰器赋予它的。</p>
<p><code>addStatic</code>里面又return了一个function，这个function的形参target就是我们的Mac类。巧妙的用了必包的方式将参数传递给了目标类。</p>
<blockquote>
<p>装饰器对类的行为的改变，是代码编译时发生的，而不是在运行时。这意味着装饰器能够在编译阶段运行代码。可以这么说，装饰器本身就是编译时执行的函数。</p>
</blockquote>
<h3 id="装饰方法"><a href="#装饰方法" class="headerlink" title="装饰方法"></a>装饰方法</h3><p>在公司的脚手架中经常看到有这样一种写法</p>
<div class="code-wrapper"><pre><code class="hljs javascript">@controller(<span class="hljs-string">&quot;/api&quot;</span>)
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">IssueTask</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">sword</span>.<span class="hljs-title">Controller</span> </span>&#123;
    @get(<span class="hljs-string">&quot;/param&quot;</span>)
    <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-title">getParam</span>(<span class="hljs-params"></span>)</span> &#123;
        ...异步请求
    &#125;
&#125;</code></pre></div>

<p>这不就是即装饰了类，又装饰了方法嘛（内心os：差点晕过去，这是什么神仙写法！）</p>
<p>装饰类我们只需关注第一个参数<code>target</code>，而装饰方法有三个参数（前两个参数没有什么luan用）</p>
<p>在之前的例子上改造一下，添加一个name方法，由于log内容太多，这里做个记号，下面单独列出。</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">getType</span>(<span class="hljs-params">type</span>) </span>&#123;
    <span class="hljs-keyword">return</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">target, name, descriptor</span>) </span>&#123;
        <span class="hljs-built_in">console</span>.log(target, name, descriptor); <span class="hljs-comment">// ①</span>
        <span class="hljs-keyword">const</span> oldValue = descriptor.value; <span class="hljs-comment">// 将之前的方法保存下来，方便下面调用</span>
        descriptor.value = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) </span>&#123; <span class="hljs-comment">// 目标方法重写了</span>
            <span class="hljs-built_in">console</span>.log(<span class="hljs-string">`我是<span class="hljs-subst">$&#123;type&#125;</span>芯片`</span>, <span class="hljs-built_in">arguments</span>); <span class="hljs-comment">// ②</span>
            <span class="hljs-keyword">return</span> oldValue.apply(<span class="hljs-built_in">this</span>, <span class="hljs-built_in">arguments</span>); <span class="hljs-comment">// 最后一定要调用之前保存的方法</span>
        &#125;
    &#125;
&#125;
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Mac</span> </span>&#123;
    price = <span class="hljs-string">&#x27;100&#x27;</span>
    @getType(<span class="hljs-string">&#x27;m1&#x27;</span>)
    <span class="hljs-function"><span class="hljs-title">name</span>(<span class="hljs-params">unit</span>)</span> &#123;
        <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;价格是：&#x27;</span>, <span class="hljs-built_in">this</span>.price + unit); <span class="hljs-comment">// ③</span>
    &#125;
&#125;
<span class="hljs-keyword">const</span> m = <span class="hljs-keyword">new</span> Mac();
m.name(<span class="hljs-string">&#x27;$&#x27;</span>);</code></pre></div>

<p>①处打印了三个形参，<code>target</code>还是类本身，<code>name</code>是装饰的方法名，<code>descriptor</code>是装饰方法的访问器属性</p>
<div class="code-wrapper"><pre><code class="hljs javascript">Mac&#123;&#125; <span class="hljs-string">&#x27;name&#x27;</span>  
&#123; 
  <span class="hljs-attr">value</span>: [<span class="hljs-built_in">Function</span>: name],
  <span class="hljs-attr">writable</span>: <span class="hljs-literal">true</span>,
  <span class="hljs-attr">enumerable</span>: <span class="hljs-literal">false</span>,
  <span class="hljs-attr">configurable</span>: <span class="hljs-literal">true</span> 
&#125;</code></pre></div>

<p>②处打印了参数列表，这里可以做一些额外的功能。arguments是④处调用时的实参</p>
<div class="code-wrapper"><pre><code class="hljs java">我是m1芯片  [Arguments] &#123; <span class="hljs-string">&#x27;0&#x27;</span>: <span class="hljs-string">&#x27;$&#x27;</span> &#125;</code></pre></div>

<p>③处是正常的打印</p>
<div class="code-wrapper"><pre><code class="hljs javascript">价格是： <span class="hljs-number">100</span>$</code></pre></div>

<p><code>注意</code>：装饰方法时，一定要把<code>descriptor.value</code>做一次保存，这么做的目的是为了装饰的目标方法不被丢失，因为我们重写了方法，这个方法里面可以做一些额为的事，并且在最后要调用那个之前被保存的目标方法，并<code>return</code>掉！</p>
<p>如果同一个类或者方法有多个装饰器，会像剥洋葱一样，先从外到内进入，然后由内向外执行。</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">dec</span>(<span class="hljs-params">id</span>)</span>&#123;
  <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;evaluated&#x27;</span>, id);
  <span class="hljs-keyword">return</span> <span class="hljs-function">(<span class="hljs-params">target, property, descriptor</span>) =&gt;</span> <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;executed&#x27;</span>, id);
&#125;
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Example</span> </span>&#123;
    @dec(<span class="hljs-number">1</span>)
    @dec(<span class="hljs-number">2</span>)
    <span class="hljs-function"><span class="hljs-title">method</span>(<span class="hljs-params"></span>)</span>&#123;&#125;
&#125;
<span class="hljs-comment">// evaluated 1</span>
<span class="hljs-comment">// evaluated 2</span>
<span class="hljs-comment">// executed 2</span>
<span class="hljs-comment">// executed 1</span></code></pre></div>

<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>装饰器就是给类或方法添加额外功能的！</p>
<h3 id="配置"><a href="#配置" class="headerlink" title="配置"></a>配置</h3><p>装饰器模式还处于提案过程，没办法在浏览器或node中直接使用，需要用<code>babel</code>转译一次</p>
<p>安装以下依赖（开发环境即可）</p>
<div class="code-wrapper"><pre><code class="hljs javascript">@babel/cli
@babel/core
@babel/preset-env
@babel/plugin-proposal-decorators</code></pre></div>

<p>新建<code>.babelrc</code>文件，添加以下内容</p>
<div class="code-wrapper"><pre><code class="hljs javascript">&#123;
    <span class="hljs-string">&quot;assumptions&quot;</span>: &#123;
      <span class="hljs-string">&quot;setPublicClassFields&quot;</span>: <span class="hljs-literal">true</span>
    &#125;,
    <span class="hljs-string">&quot;plugins&quot;</span>: [
      [<span class="hljs-string">&quot;@babel/plugin-proposal-decorators&quot;</span>, &#123; <span class="hljs-string">&quot;legacy&quot;</span>: <span class="hljs-literal">true</span> &#125;],
      [<span class="hljs-string">&quot;@babel/plugin-proposal-class-properties&quot;</span>]
    ]
&#125;</code></pre></div>

<p>修改<code>package.json</code>文件</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-string">&quot;scripts&quot;</span>: &#123;
     <span class="hljs-string">&quot;test&quot;</span>: <span class="hljs-string">&quot;echo \&quot;Error: no test specified\&quot; &amp;&amp; exit 1&quot;</span>,
 +   <span class="hljs-string">&quot;build&quot;</span>: <span class="hljs-string">&quot;babel index.js -d dist&quot;</span>
 &#125;,</code></pre></div>

<p>完成后直接<code>npm run build</code>命令即可</p>
<p>babel会将index.js文件转译一次，输出到<code>dist</code>文件夹下，直接<code>node</code>dist下的文件就好了</p>
<p>以上配置只能用与学习，不能用与实际开发中！</p>

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